Tribological performance of polyoxymethylene (POM): analysis of temperature effect and evaluation of increasing load

Polymers are advanced materials found in almost every product of our daily life. To date, the importance of plastics has been highlighted as a result of their widespread use in different sectors of sciences, technologies and industry, from basic employments until medical and therapeutic applications. In the present scenario, because of their self-lubricating properties, polymers are extensively adopted for manufacturing sliding components (gears, bearings, cams, roller in conveyors, etc.) which undergo wear and tear. Along with their broad utilization for tribological purposes, the understanding of tribology of polymers is becoming increasingly relevant. This study is focused on a specific engineering thermoplastic material, the polyoxymethylene (POM). Due to its ability to combine good mechanical properties with excellent lubrication characteristics, nowadays polyoxymethylene is gaining significant importance. The aim of the work was to evaluate the tribological performance of POM sliding against different metal counterfaces. The two main parameters that have been investigated, under different conditions and by means of diverse methods, were temperature and load. The first set of experiments involved the simulation of a rotary movement between a polymeric ball and an AISI 52100 steel disk. The instrument that allowed the testing was a Rotary Tribometer and the sliding contact was achieved in dry conditions. The results arising from gradually increasing the temperature were compared and, therefore, the temperature effect was examined.The second type of analysis, accomplished through a BioTribometer without any lubricant, was carried out putting in reciprocating contact a polymeric ball and a cobalt-based alloy disk, whether covered or not with a titanium nitride coating. The mechanism occurring at the interface was observed by reproducing the tests with different load forces and by processing the consequent outcomes.The conclusions arisen from these two kind of tests really differed from what was expected: the formation of a transfer layer, phenomenon generally exhibiting when a polymeric material slides against a metal, did not take place. Assuming that the coating made of transferred material acts as an insulating barrier between the two surfaces, an enhancement of tribological performance is predictable, and, accordingly, the absence of this layer should lead to a worsening of polymer’s properties. Nonetheless, a certain percentage of improvement was detected in both cases.Experiments conducted at growing heating indicated the worst results at the intermediate temperature (45°C), while the best ones were reported at the highest temperature (70°C). This condition suggested that the temperature parameter is beneficial only if its increment is suitably appreciable.Tests addressed to investigation of load effect showed on the one hand a direct proportionality between rising load and wear rate, on the other hand a lowering of friction over mounting load. Both wear and friction associated with TiNbN-coated disks turned out to be more severe than with the uncoated ones. The optimal condition was determined under a 250 N applied load.